Migration flows are shaped by a complex combination of self-selection and out-selection mechanisms. In this paper, we analyze how existing diasporas (the stock of people born in a country and living in an another one) affect the size and human-capital structure of current migration flows. Our analysis exploits a bilateral data set on international migration by educational attainment from 195 countries to 30 OECD countries in 1990 and 2000. Based on simple micro-foundations and controlling for various determinants of migration, we find diasporas increase migration flows, lower their average educational level and lead to higher concentration of low-skill migrants. Interestingly, diasporas explain majority of the variability of migration flows and selection. This suggests that, without changing the generosity of family reunion programs, education-based selection rules are likely to have moderate impact. Our results are highly robust to the econometric techniques, accounting for the large proportion of zeros and endogeneity problems.